Compressors currently in use can generally be divided into two major categories, reciprocating and centrifugal. Reciprocating compressors reduce the volume of gas to increase pressure, while centrifugal compressor increase the kinetic energy of the gas and then convert the kinetic energy into increased pressure.
Advantages of centrifugal compressors over reciprocating compressors include lower sensitivity to gas purity, mechanically simpler operation and lower vibration levels. The major disadvantage of centrifugal compressors is that the attainable pressure rise is generally limited. Centrifugal compressors are also difficult to miniaturize it is due to its working principle, the pressure rise being dictated by the tip speed of the impeller which is mounted on a rotating shaft. If a small impeller is used, the impeller must be spun faster to keep the same tip speed and hence the same pressure rise. Reducing the size of the impeller to several centimeters causes the impeller to spin at several hundred thousand RPM. At this speed, problems including centrifugal stress in the rotor, as well as problems with the bearings, driving method and cooling issues all generally arise.
For high speed machines, gas lubricated bearings are mostly used. Such bearings can be classified under two categories namely, aerostatic and aerodynamic. The aerostatic type requires additional gas flow to function, which significantly reduces compressor efficiency, especially for miniature compressors which provide power levels on the order of only a few kilowatts. Aerodynamic type gas bearings can be fit into a small space and can be used without any additional gas supply. However, aerodynamic type gas bearings demand extremely high operational speeds in order to function properly and can cost about $40,000/pair to design, fabricate and test them. Superior aerodynamic bearing technology comprises of magnetic bearings, flexure pivot tilting pad bearings and foil bearings, etc. Magnetic bearings, which require relatively large space to install and complicated driving circuits, are difficult to shrink to fit into applications in which space is limited, such as for a miniature compressor. Tilting pad bearings that work satisfactorily above 100,000 RPM are difficult to find and are also costly. Gas foil bearings are generally too expensive (about $90,000/pair to design, fabricate and test) to use for a low-cost kilowatt level compressor. Gas foil bearings generally only have applications for relatively high-value applications, such as for aircraft engines.
As described above, to handle the high speed impeller rotation required by miniature compressors providing power levels on the order several kilowatts, available centrifugal compressors all use some type of non-contact bearing to handle the high speed. Although certain bearing embodiments can provide some advantages, disclosed bearing embodiments for miniature compressors which provide power levels on the order several kilowatts are typically expensive to implement and sensitive to the working environment. What is needed is a compact, high efficiency, high reliability centrifugal compressor design that provides kilowatt level power output, that is also economical to produce.